High-pressure water pumps which utilize piston and cylinder arrangements are known and the type of high-pressure water pump with which the present invention is concerned comprises at least one cylinder, a cylinder bushing or sleeve within this cylinder, a cylinder head, a metal piston reciprocatable in the cylinder bushing and a piston shoe on the piston and engageable with an eccentric carried by an eccentric shaft journaled in an eccentric shaft housing.
The pump further comprises intake and outlet valves with valve closure members and the piston guide shoe operatively connects the piston with the eccentric so that upon rotation of the eccentric shaft, the eccentric will reciprocate the piston to alternately expand and contract the cylinder compartment or chamber defined between the piston and the cylinder head in the cylinder bushing During an intake stroke, corresponding to expansion of the cylinder chamber, a low pressure is developed in the cylinder chamber and water is drawn from the eccentric shaft compartment into the cylinder chamber During the succeeding stroke, namely the discharge stroke, the volume of the cylinder chamber is contracted and the water is forced under high pressure from the cylinder chamber.
To supply the water, a low-pressure reservoir is generally provided and can be connected to the housing by an appropriate flange communicating between the eccentric shaft compartment and the low-pressure water reservoir. For the purposes of this application, low pressure means a water pressure of 10 bar or less. The water is drawn out of the eccentric shaft compartment via at least one intake valve during the intake stroke into the cylinder chamber. The intake valve opens when the water pressure in the cylinder chamber is below the low pressure of the reservoir by a predetermined low-pressure threshold.
If the pressure difference is smaller than the low pressure threshold or with an opposite sign, the suction valve is closed.
During the displacement stroke, the water in the cylinder chamber is compressed at high pressure. For the purposes of this application, the term high pressure means a water pressure of, for example, 60 bar to 450 bar.
The outlet valve opens as a rule at a selectable high-pressure threshold of the water pressure, which corresponds to the desired minimum high-pressure level. Below this high-pressure threshold, the outlet valve is closed. Upon exceeding the high-pressure threshold during the displacement stroke, the outlet valve opens to permit the displaced water to flow to the outlet port of the housing under high pressure.
The kinematics of the piston movement is such that the piston has a so-called upper dead point and so-called lower dead point The stroke of the piston is established by the rotation of the eccentric which is coupled to the piston by the piston guide shoe which pushes the piston toward the upper dead point position or allows the movement of the piston, e.g. under spring force, into the lower dead point position. A spring can therefore retain the shoe of the piston against the eccentric.
The piston can be guided, in its lower dead point position, over its entire length in the cylinder bushing or can have a portion of the piston turned toward the eccentric shaft which is withdrawn from the cylinder bushing in its lower dead point position. If the piston and cylinder bushing are of the same length, the piston in its lower dead point position is guided in the cylinder bushing over a length which is equal about to the difference between the length of the cylinder bushing and the piston stroke. In any event, the piston and cylinder bushing should be dimensioned with respect to their lengths and the stroke such that detrimental canting of the piston does not occur in operation.
In high-pressure pumps of the aforedescribed type provided heretofore, both the piston and the cylinder bushing were composed of metallic materials. A clearance was frequently defined between the piston and cylinder bushing which would allow sliding of the piston in the cylinder bushing at the operating temperature range. In other words at the operating temperature, with thermal expansion, the tolerance was such that the piston was not permitted to seize in the cylinder. The length over which the piston is guided in the cylinder could be defined as the gap length.
In high-pressure water pumps, the water which is displaced has functional significance for the operation of the pump. On the one hand, the high-pressure pump is continuously cooled by the water flow through it. On the other hand, the displaced water also performed a lubricating function since it generally carried a lubricant along with it. Free slidable surfaces of the pump were continuously wetted with the lubricant carried by the water. Indeed, lubricant content of the water could be as much as 5%, although lesser lubricant contents could be used.
When both the piston and the cylinder bushing were composed of metal, a minimum lubrication was essential. Should the supply of lubricant to these surfaces be reduced below the necessary minimum, the temperature of the cylinder bushing and the piston would rise because of increased friction and in spite of the above-mentioned cooling effect. With increased friction, there was increased wear of material from the piston and/or the bushing which resulted in increasing detriment to the function of the high-pressure water pump. In practice it was found that the conventional high-pressure water pumps, operated without the addition of lubricant to the water, had a relatively short life and rapidly deteriorated for the reasons given above. However, the lubricants used were detrimental to the environment if the displaced water was not conducted in a closed path.
In most cases in which high-pressure water is used, a closed path for the water is impossible or, at best, is extremely expensive. In other words, lubricant addition is undesirable on environmental grounds but is a practical necessity on technological grounds for effective operation of the high-pressure water pump.